Combine vinegar, 1½ cups (375ml) water, the sugar, salt and pepper in slow cooker. Place beef in centre of cooker, fat-side up; arrange cabbage around beef. Cook, covered, for 6 hours. Transfer beef to a large plate or tray; rest for 10 minutes then cut into thin slices. Drain cabbage.
Meanwhile, just before serving, heat oven grill on high. Spread bread slices with softened butter; grill for 1 minute on each side. Top half the bread slices with cheese; grill for 1 minute or until melted. Divide sliced corned beef among melted cheese toasts, then top with cabbage and remaining bread slices.